Leading race horse Australia, which won both this year's English Derby and the Irish Derby, has been retired just a week before the colt was due to bow out at the British Champions Day meet at Ascot, it was announced Saturday.

According to a UK representative of the Ireland-based Coolmore Stud, where Australia was trained by Aidan O'Brien, the colt has suffered a hoof problem which means racing next Saturday is no longer an option.

"Unfortunately Australia developed a problem in his right-hind hoof during the week." Kevin Buckley, UK representative for Coolmore, told Channel 4 Racing,

"Following consultation between the resident farrier at Ballydoyle, our vet, John Halley, and the respective owners, the decision was made yesterday to retire him to Coolmore."

The three-year-old won five races, with his career yielding more than STG2 million ($A3.65 million) in prize money, but his last appearance ended in defeat being beaten by French Derby winner The Grey Gatsby in the Irish Champion Stakes last month.